1. Field of the Invention
This invention pertains to electrostatic devices generally, and in one specific preferred embodiment pertains to arc and fault prevention in electrostatic coating processes.
2. Description of the Related Art
Electrostatic devices have been used in a wide array of applications and environments. Electrostatic paper handlers and toner transfer systems are found in office equipment such as copiers and printers. In a manufacturing environment, many painting and coating processes are very dependent upon performance achievable only with the electrostatic process. A common feature of primary applications for electrostatic devices is the need to move or transfer material as perfectly as possible from one location to another. In painting or coating systems, a source feed is provided to the device. The feed material may be a liquid or powder, and it may be heated, be at ambient or even be cooled. The source feed is generally mixed with gas and expelled from a nozzle. During expulsion, particles within the mixed feed are charged electrically with a substantial voltage potential. A receiving surface is also charged to a high potential, but of opposite polarity, or to ground as is generally done in the automobile industry. As is known, the voltage differential provides substantial force of attraction between the charged particles and receiving surface. As a result, the charged particles are drawn to the receiving surface, even if they were otherwise propelled in a different direction.
For those unfamiliar with the strength of electrostatic forces, the Common grade school trick of rubbing a balloon against one""s hair provides force ample to maintain the balloon suspended against the ceiling, even when the balloon itself is well beyond providing any significant buoyancy. Often, electrostatic forces will cause a person""s hair to literally stand on end. These events occur with large objects, such as hair and balloons, that are relatively massive when compared to fine particles emitted from an atomizer, fine sprayer or the like. Consequently, electrostatic forces have proven to be valuable when working with relatively fine particles, and, in some instances, even when working with larger objects. Unfortunately, several drawbacks are present in the prior art systems that tend to limit the application of this technology. Electric arcs are very destructive. Energy density within an arc is so great that an extremely high temperature plasma is formed. There are very, few materials that are resistant to a plasma, and, as a result, plasma from an arc can destroy not only the coating material, but also the product which is being coated, and the high dissipation of energy can be adverse to the electrostatic device as well. An electric arc is also a fire and explosion hazard, even with materials that would not normally be considered to be particularly flammable or explosive. In electrostatic coating processes such as paint spraying, many paints have some level of combustibility and some are highly combustible. As a result of the risk of explosion, many painting applications are not considered suitable for electrostatic processes. These problems related to arcing may be her compounded by the geometry of the product being coated. All too often, the product to be coated is quite irregular in shape. A sharp edge, a pointed region, or even just a significant protrusion can greatly increase the chances of a voltage breakdown. Similarly, reducing the distance between the electrostatic sprayer and the work too much, such as might occur during a brief lapse in operator concentration, will result in a breakdown of the dielectric, and an arc will result. Beyond arcing, electrostatic devices have traditionally had another shortcoming. When the device or equipment is powered up or turned on, the device may electrically be short-circuited. This can happen when the operator puts the device down in order to switch power on, or when, at the of the previous work day the operator set the device down in a poor location, Or it can happen if the device is accidentally displaced, such as by being bumped or accidentally dropped. When the operator starts the electrostatic device, the device immediately overloads, in some cases destroying the equipment and also the work surface. Unfortunately, the operator may only then realize that the device was resting against an electrical ground. All components involved in this unfortunate situation, including the power supply, sprayer, and underlying component substrates, may be damaged. Faced with this dilemma, there have been some alternatives proposed, none which are fully satisfactory. For example, a first alternative is to use a robotic device designed to carry an electrostatic sprayer. The robotic device is programmed to work with a specific product where it will follow a specific surface contour. This requires an initial, expensive purchase of the robot, and further requires significant programming before the robot may be used. Nevertheless, if the robotic device is installed in a high volume production line, the expense and time incurred may be justifiable when amortized across many parts to be produced.
Unfortunately, the distance between the electrostatic coating device and work, and geometry of the work are only two factors that affect likelihood of arcing. Ambient conditions such as temperature and humidity are also known to have a substantial effect, such that on any given day, the probability for arcing may be higher or lower, and may also depend upon the source feed. Changes in source feed from a manufacturer may be difficult or impossible to predict or control, and the ambient conditions may also be difficult or impossible to control, depending upon the particular manufacturing environment. As a result, robotic systems have found greatest application in consistent or artificially controlled environments; where volumes are high enough to offset the difficulty and expense of acquiring and programming the robot; and where variances in source feed or ambient have a minimal impact on the operation of the device, such as with a few select materials and products.
A second alternative that reduces the likelihood of arcing is to simply reduce the voltage supplied to the electrostatic device. If the voltage is reduced, the probability of an inadvertent arc is also reduced. However, simply reducing the voltage does not help where a higher voltage is desirable or necessary for effective coating. Furthermore, lowering the voltage does not help an operator who may be traversing a flat surface without problem, who unknowingly encounters an unforeseen sharp protrusion. As aforementioned, a sharp protrusion or edge will greatly increase the probability of an arc. Therefore, there is a great need for an electrostatic power source which is capable of preventing destructive electrical arcing and which will protect the system against accidental short-circuiting. Unfortunately, in many applications of the prior art, the substantial benefits of the electrostatic device have remained heretofore untapped.
In a first manifestation of the invention, a computerized apparatus controls power supplied to a high-voltage, low current output electrostatic device. The apparatus provides intelligent fault detection and arc prevention, and is readily adjusted and adapted to varying environmental conditions and various applications. A user interface is provided for entering control parameters and receiving status indication, and may take the form of various display panel components, serial communications channels, Ethernet interfaces or other suitable arrangements. A variable power supply is provided with a control input and a variable voltage output, and an electrical conductor electrically interconnects the variable voltage output to the electrostatic device. A means for sensing the variable voltage output and a means for sensing high-voltage, low current output from the electrostatic device each provide an output signal indicative of magnitude to a microprocessor system. Using both hardware and programs in response to output voltage, output current, and control parameters, the microprocessor system adjusts the variable power supply through the control input to remediate fault conditions, thereby preventing arcs and other damaging fault conditions from adversely affecting the computerized apparatus.
The invention also comprises a method for preventing arcs or destructive fault conditions from damaging an electrostatic device.
Power is provided to the electrostatic device for high voltage applications and includes a current component of the high voltage. Predetermined limits are established for a rate of change of the current component. At predetermined time intervals, a timing interrupt signal is generated. When the interrupt signal is generated, the current component is measured, and a rate of change of current with respect to time is calculated. When the calculated rate of change of current is outside of the predetermined rate of change limits, power is discontinued (dI/dt Fault) or reduced (dI/dt Foldback FB) to the high voltage generator.
In a third manifestation, the invention is a power source for an electrostatic sprayer used to electrostatically spray a coating onto a substrate, wherein the coating is attracted to substrate by strong electrostatic forces to produce an improved coating with reduced over-spray. The power source protects from undesirable electrical arcing and electrical faults. A high electrostatic voltage output is generated by a voltage multiplier that converts a low voltage into high electrostatic voltage. The low voltage is derived from a low-voltage variable power supply that includes a control input. A fault relay is included for alternatively connecting and disconnecting the low voltage from voltage multiplier. A Current sensor provides a signal representing an amount of current flowing through the high electrostatic voltage output, and a voltage sensor provides a signal representing a magnitude of the Low voltage. Acceptable values for current signal and voltage signal are also established. A microprocessor subsystem includes hardware and software algorithms for providing a control signal to the Low-voltage variable power supply control input responsive to deviations of current signal and voltage signal from established values, and is isolated from the high electrostatic voltage output and low voltage variable power supply. As a result, the microprocessor subsystem is able to safely control the Low voltage variable power supply when the low voltage power supply is destructively energized.
A first object of the invention is to provide a microprocessor controlled apparatus and method for controlling a high-tension power supply. A second object of the invention is to ensure that the high-tension power supply is highly resistant to arcing. Another object of the invention is to allow an operator to control or set limits within the power supply to enable the power supply to be used with very diverse materials and, where required, in very diverse environments. A further object of the invention is to eliminate start-up faults that would otherwise be damaging to the power supply, operator, or other objects or persons. Yet a further object of the invention is to protect the equipment and operator against accidental or inadvertent short circuits. These and other objects of the invention are achieved in the preferred embodiment, which offers significant advantage over prior art systems.